Breast cancer is the leading type of cancer in women and is the second leading cause of cancer death among women. Based on the current life expectancy for women in the United States, 1 out of 9 women will develop breast cancer in her lifetime - a risk that was 1 out of 14 in 1960. Tumor metastasis still remains the main cause of breast cancer death. Although with chemotherapy and radiation therapy, the prognosis has improved in some cases, these approaches often result in severe side effects. Two proteins - JAK2 and PAK1-have been implicated in breast cancer. Tyrosine (Tyr) kinase JAK2 was identified as a prolactin receptor-bound signaling molecule. The prolactin receptor is detected in 80% of human breast cancers and is overexpressed in breast cancer cells. Normal and tumor mammary epithelial cells synthesize prolactin and prolactin receptor, thus the prolactin could behave as an autocrine growth factor for human breast cancer cells. Serine-threonine kinase PAK1 has been linked to breast cancer by several lines of evidence: (a) PAK1 gene amplification and/or PAK1 protein up-regulation have been reported in breast cancers; (b) activated PAK1 increased cell invasion of breast cancer cells; (c) PAK1 is involved in the activation or regulation of several distinct mitogen-activated protein kinase cascades that lead to increased proliferation; [unreadable] (d) PAK1 contributes towards cancerous phenotypes by enhancement of cell survival. Preliminary data [unreadable] demonstrate that PAK1 associates with and is Tyr phosphorylated by JAK2. Two-dimensional peptide [unreadable] mapping identified three Tyr(s) of PAK1 which are phosphorylated by JAK2. Tyr phosphorylation of PAK1 by JAK2 was also shown to protect cells from apoptosis and increase cell motility. This grant proposes to examine the hypothesis that JAK2-dependent tyrosyl phosphorylation of PAK1 activates PAK1, increases cell proliferation, and causes anchorage-independent growth and invasiveness of human breast cell. Aim1 will determine whether JAK2 phosphorylation of PAK1 alters in vitro growth properties of human breast cells by promoting cells proliferation and/or anchorage-independent growth. Aim2 will determine whether JAK2 phosphorylation of PAK1 causes human breast cell invasiveness by increasing cell motility and/or decreasing cell adhesion. Although the significance of both JAK2 and PAK1 in breast cancer is widely acknowledged, the mechanism involved remains poorly understood. There is a gap between upstream JAK2-dependent events and downstream PAK1 and PAK1-dependent functions in our understanding of the mechanism of breast cancer progression. Exciting preliminary data suggest that PAK1 is an important signaling molecule phosphorylated by JAK2 and participating in cell survival. The results of the proposed studies will provide important insight into the fundamental mechanism by which tyrosyl phosphorylation of PAK1 by JAK2 regulates cell proliferation and invasiveness. It will also provide needed insight into the possible mechanism by which JAK2 and PAK1 participate in breast cancer. We believe that tyrosyl phosphorylation of PAK1 by JAK2 is likely to represent a novel molecular target in the search for theetiology and treatment of human breast cancer. [unreadable] PUBLIC HEALTH RELEVANCE: Breast cancer is the most commonly diagnosed cancer in women - nearly 1 in 3 (30%) of all cancers in women occur in the breast. Based on the current life expectancy for women in the United States, 1 out of 9 women will develop breast cancer in her lifetime - a risk that was 1 out of 14 in 1960. Much is still unknown regarding the molecular biological mechanism by which a normal cell becomes a cancer cell. Normal cell behavior is tightly controlled by multiple signaling pathways that ensure that cells proliferate only when they are required by the body. Cancer occurs when normal growth regulation breaks down, usually because of a defect in these signaling mechanisms. Metastasis, the spread of cancer to distant sites in the body, is in fact what makes cancer so lethal. A surgeon can remove a primary tumor relatively easily, but a cancer that has metastasized usually reaches so many places that cure by surgery alone becomes impossible. The ability of cells to metastasize is also regulated by multiple signaling pathways. Two proteins - JAK2 and PAK1 -have been implicated in the regulation of cell pathways that can lead to breast cancer but how they work together and the precise mechanism of their action is unknown. Our long term goal is to understand the molecular mechanisms that control cell division and cell motility and disregulation of which leads to human breast cancer. Toward this aim, we have started to analyze the relationship between JAK2 [unreadable] and PAK1. We showed that JAK2 binds to PAK1 and makes PAK1 more active. Activated PAK1 contributes to better cell survival that may promote cancer development. In the current proposal we will study how JAK2 and PAK1 increase cell proliferation and cell migration that leads to metastasis. In understanding of how these proteins work together will help to design new therapeutic approaches and possibly drugs for treatment of human breast cancer. [unreadable] [unreadable] [unreadable]